The methylation of a nickel(II) complex (CTPP)Ni-II of 2-aza-5,10,15,20-tetraphenyl-21-carbaporphyrin (CTPPH2) with the mild methylating agent methyl iodide yields novel stable organonickel(II) complexes : diamagnetic (21-CH3TPP)Ni-II and two scarce paramagnetic species (2-NH-21-CH3CTPP)Ni(II)X and (2-NCH3-21-CH3CTPP)Ni(II)X (X = Cl, I). The demetalation procedure resulted in isolation of two 21-carbaporphyrin derivatives, methylated at the internal C(21) carbon atom, i.e., 2-aza-5,10,15,20-tetraphenyl-21-methyl-21-carbaporphyrin (21-CH3-CTPPH2) and 2-aza-2-methyl-5,10,15,20-tetraphenyl-21-methyl-21-carbaporphyrin (2-NCH3-21-CH3CTPPH). The methylation mechanism involves the oxidative addition of the methyl cation to the carbaporphyrin C(21) activated due to Ni-C coordination. These C-methylated macrocycles preserve their coordinating properties as the remetalation processes have been carried out. The reversible concerted addition of HX converts the diamagnetic (21-CH3TPP)Ni-II into the paramagnetic (2-NH-21-CH3CTPP)Ni(II)X. The axial coordination step is accompanied by a protonation of the peripheral nitrogen. The structures of (21-CH3CTPP)NIII (monoclinic, P2(1)/c; a = 15.055(3) Angstrom, b = 15.795(3) Angstrom, c = 17.069(3) Angstrom, beta = 115.00(3)degrees, Z = 4, least-square refinement of 476 parameters using 3622 reflections, R = 0.069) and (2-NCH3-21- CH3CTPP)(NiI)-I-II (monoclinic, P2(1)/n; a = 12.946(3) Angstrom, b = 23.519(5) Angstrom, c = 13.945 Angstrom; beta = 93.2(3)degrees, Z = 4, the least-square refinement of 502 parameters using 3257 reflections, R = 0.0559) have been determined by X-raji diffraction. In the first case the nickel(II) is four-coordinate with bonds to three pyrrole nitrogen atoms (Ni-N distances 1.948(5); 1.955(5); 1.938(5) Angstrom) and the pyrrole carbon (Ni-C 2.005(6) Angstrom). The nickel lies in the plane of the three nitrogens, while the methylated pyrrole is sharply tilted out of the plane with dihedral angle between the plane of three nitrogens and the methylated pyrrole plane being -42.2 degrees. The methylated pyrrole is bound to nickel via a pyramidal carbon in the eta(1)-fashion. The first structurally characterized paramagnetic organonickel(II) complex (2-NCH3-21-CH3CTPP)(NiI)-I-II presents unique features related to its electronic structure. The nickel(II) is five-coordinate with bonds to three pyrrole nitrogen atoms (Ni-N distances 2.032(8); 2.057(8); 1.979(8) Angstrom) and to the pyrrole C(21) carbon (Ni-C 2.406(9) Angstrom). : diamagnetic (21-CH3TPP)Ni-II and two scarce paramagnetic species (2-NH-21-CH3CTPP)Ni(II)X and (2-NCH3-21-CH3CTPP)Ni(II)X (X = Cl, I). The demetalation procedure resulted in isolation of two 21-carbaporphyrin derivatives, methylated at the internal C(21) carbon atom, i.e., 2-aza-5,10,15,20-tetraphenyl-21-methyl-21-carbaporphyrin (21-CH3-CTPPH2) and 2-aza-2-methyl-5,10,15,20-tetraphenyl-21-methyl-21-carbaporphyrin (2-NCH3-21-CH3CTPPH). The methylation mechanism involves the oxidative addition of the methyl cation to the carbaporphyrin C(21) activated due to Ni-C coordination. These C-methylated macrocycles preserve their coordinating properties as the remetalation processes have been carried out. The reversible concerted addition of HX converts the diamagnetic (21-CH3TPP)Ni-II into the paramagnetic (2-NH-21-CH3CTPP)Ni(II)X. The axial coordination step is accompanied by a protonation of the peripheral nitrogen. The structures of (21-CH3CTPP)NIII (monoclinic, P2(1)/c; a = 15.055(3) Angstrom, b = 15.795(3) Angstrom, c = 17.069(3) Angstrom, beta = 115.00(3)degrees, Z = 4, least-square refinement of 476 parameters using 3622 reflections, R = 0.069) and (2-NCH3-21- CH3CTPP)(NiI)-I-II (monoclinic, P2(1)/n; a = 12.946(3) Angstrom, b = 23.519(5) Angstrom, c = 13.945 Angstrom; beta = 93.2(3)degrees, Z = 4, the least-square refinement of 502 parameters using 3257 reflections, R = 0.0559) have been determined by X-raji diffraction. In the first case the nickel(II) is four-coordinate with bonds to three pyrrole nitrogen atoms (Ni-N distances 1.948(5); 1.955(5); 1.938(5) Angstrom) and the pyrrole carbon (Ni-C 2.005(6) Angstrom). The nickel lies in the plane of the three nitrogens, while the methylated pyrrole is sharply tilted out of the plane with dihedral angle between the plane of three nitrogens and the methylated pyrrole plane being -42.2 degrees. The methylated pyrrole is bound to nickel via a pyramidal carbon in the eta(1)-fashion. The first structurally characterized paramagnetic organonickel(II) complex (2-NCH3-21-CH3CTPP)(NiI)-I-II presents unique features related to its electronic structure. The nickel(II) is five-coordinate with bonds to three pyrrole nitrogen atoms (Ni-N distances 2.032(8); 2.057(8); 1.979(8) Angstrom) and to the pyrrole C(21) carbon (Ni-C 2.406(9) Angstrom).